Both seem to be of the view the US has neither the will nor the consistency to put up resistance. Indeed, Trump's White House gave evidence of exactly that this week, floundering for a coherent response to allegations Russia paid the Taliban to kill US forces in Afghanistan. The Russian Embassy in Washington and the Taliban have both denied the claims.

According to White House deputy press secretary Sarah Matthews: "President Trump is a world-class negotiator who has consistently furthered America's interests on the world stage."

Former senior US officials provided CNN contributor Carl Bernstein with a very different take. They thought Trump "delusional" about his ability to bend other leaders to his agenda, believing he could "either charm, jawbone or bully almost any foreign leader into capitulating to his will."

But Putin "just outplays him," said one of Bernstein's sources. Trump's "fawning over authoritarian strongmen, his ignorance of history and lack of preparation" jeopardized US national security, Bernstein was told.

Whether it's Putin's payback or Xi's decision to violate and chip away at the Hong Kong agreement signed with the UK in 1984, both leaders appear to see opportunities.

Go back three years. Russian Foreign Minister Sergey Lavrov dropped into the Oval Office the day after Trump fired James Comey as FBI director. Comey was overseeing the investigation into allegations of Russian election meddling. An official Russian photographer caught the bonhomie, as Trump told his visitors: "I just fired the head of the FBI. He was crazy, a real nut job."

Two months later, Trump met Putin -- alone -- on the margins of the G20 summit. The White House trumpeted the encounter as a success, highlighting a new ceasefire deal in Syria. The Russians cynically used the deal to freeze the conflict, allowing the Assad regime backed by Russia to pick off rebel-held areas one-by-one.

Trump could have protested, ripped up the deal, forged a new US policy on Syria that would have crimped Russia's growing influence in the Middle East. Instead he bought the lie.

It would be a recurring theme. Trump's former national security adviser John Bolton recalls in his new book "The Room Where It Happened" Trump's 2018 meeting with Putin in Helsinki, Finland.

"Putin had to be laughing uproariously at what he had gotten away with," writes Bolton, after Trump accepted Putin's assurances there had been no Russian interference in the 2016 US campaign.

So fast forward to this year as Putin, in power for 20 years, needed a constitutional fix to hold on for longer. He appears to calculate that despite tightened US sanctions over his invasion of Ukraine, and the poisoning of exiled former agent Sergei Skripal in the UK, Trump won't push back for what he does at home.

He was right. He is now effectively President for life, hoping to emulate Soviet leaders by stepping down only into his grave.

Xi's experience with Trump has been different. Tangled in a trade war, he has had to calculate Trump's real intent: on money, human rights issues, such as the Uyghurs or Hong Kong, and simply stopping the rise of the world's next superpower.